1. Field of the Invention
This invention relates to a regeneration system for an exhaust gas cleaning device, and more particularly to a regeneration system comprising an exhaust gas cleaning device disposed in an exhaust emission path of an internal combustion engine and provided with an exhaust gas cleaning honeycomb filter for catching particulates included in the exhaust gas and a heating means for the exhaust gas cleaning honeycomb filter when a fuel containing a fuel additive for mitigating particulates included in the exhaust gas is used as a fuel for the internal combustion engine.
2. Description of Related Art
In the internal combustion engine such as diesel engine or the like, particulates (e.g. soot or unburned portion of fuel) are included in the exhaust gas. Particularly, the discharge amount of particulates becomes large in a diesel engine using a gas oil as a fuel or a direct-injection type gasoline engine recently coming into wide use. Therefore, it is well-known to remove the particulates by an exhaust gas cleaning device disposed in an exhaust emission path of the internal combustion engine and provided with an exhaust gas cleaning honeycomb filter.
As the exhaust gas cleaning honeycomb filter is generally used a cordierite filter 32 of a honeycomb structure as shown by a diagrammatically section view in FIG. 1. In such a conventional cordierite filter 32 are included a plurality of exhaust gas flowing channels 33 extending in parallel to a longitudinal direction thereof, wherein these channels 33 are alternately plugged at either upstream side or downstream side for the exhaust gas of their ends with plugging members 33a to form a checker pattern.
As shown in FIG. 1, an exhaust gas Gin emitted from a diesel engine (not shown) flows into the cordierite filter 32 through the exhaust emission path 11, at where particulates included in the exhaust gas are filtered off on surfaces of cell walls constituting the exhaust gas flowing channels 33. Then, the cleaned exhaust gas Gout passed through cordierite filter 32 again passes through the exhaust emission path 11 and is discharged out to the outside of the vehicle.
It is known that pressure loss xcex94P is produced when the exhaust gas Gin passes through the filter 32. The pressure loss xcex94P is represented by the following equation (1).
xcex94P=xcex94P1+xcex94P2+xcex94P3+xcex94P4xe2x80x83xe2x80x83(1)
wherein
xcex94P1 is a resistance produced due to the narrowing of an opening portion in the exhaust gas flowing channel 33 when the exhaust gas flows into the channel 33 through the exhaust emission path 11;
xcex94P2 is a resistance produced in the flowing of the exhaust gas through the exhaust gas flowing channel 33;
xcex94P3 is a resistance produced in the passing through a wall of the exhaust gas flowing channel 33;
xcex94P4 is a resistance produced when the exhaust gas passes through particulates deposited on the surface of the exhaust gas flowing channel 33.
In this case, the resistances xcex94P1, xcex94P2, xcex94P3 are dependent upon a cell structure constituting the filter 32, respectively and are a constant value xcex94Pi not depended upon the lapse of time coming into problem in the deposition of the particulates and the like (hereinafter xe2x80x9cxcex94P1+xcex94P2+xcex94P3xe2x80x9d is called as xe2x80x9cinitial pressure lossxe2x80x9d). For this end, a greater part of the pressure loss xcex94P is determined by the resistance xcex94P4 produced when the exhaust gas passes through the particulates deposited on the cell walls. The resistance xcex94P4 is usually 2-3 times the initial pressure loss xcex94Pi at a deposited state of the particulates.
In FIG. 2 is shown a relation among cell structure, typical dimensions, geometrical surface area and opening ratio in the filter. The cell structure Cs (mil/cpi) is represented by thickness of cell wall dc (mil=milli inch) to cell number Nc per square inch (cpi=cells per square inch), and the geometrical surface area fs (cm2/cm3) is an area passing the exhaust gas per unit volume (filtering area). Moreover, the cell wall thickness dc is shown by unit of mm in FIG. 2.
As seen from FIG. 2, the pressure loss xcex94P produced in the checkered honeycomb filter for cleaning the exhaust gas is small as the cell number Nc and geometrical surface area fs in the filter become large. And also, the opening ratio xcex1 (%) is a ratio of total opening area of the exhaust gas flowing channels occupied in the sectional area of the filter. As shown in FIG. 2, a limit not creating cracks (crack limit) is large as the opening ratio a becomes small.
On the other hand, a mechanical strength of the filter, i.e. bending strength S* of the filter is approximately equal to product of strength S of a filter made of porous material and relative density xcfx81* as mentioned below. When the nature of the porous material constituting the filter is represented by density xcfx81 and strength S, the bending strength S* of the filter and the relative density xcfx81* are as follows:
xcfx81*=xcex1xc3x97xcfx81xe2x80x83xe2x80x83(2)
S*≅xcfx81*xc3x97S xe2x80x83xe2x80x83(3)
That is, the strength is high as the opening ratio xcex1 becomes small.
Further, the regeneration of the filter is carried out by burning the particulates according to the following reaction equation (4):
C+O2xe2x86x92CO2+Q (heat quantity)xe2x80x83xe2x80x83(4),
so that the strength of the filter against the thermal stress becomes important. Particularly, when the filter is made from a ceramic material, brittle rupture is caused by thermal stress to create cracks. Such a cracking phenomenon is apt to be created as heat quantity produced in the regeneration or the amount of the particulates deposited to be burnt becomes large. Moreover, the unburned portion of fuel constituting the particulate is an organic compound, so that it is burned by heating the filter. As mentioned below, a crack limit preventing the occurrence of the cracking phenomenon is proportional to the opening ratio xcex1 and is closely related to the thickness dc of the cell wall as seen from FIG. 2. If the opening ratio xcex1 is same, as the thickness dc of the cell wall becomes thick, the crack limit is high.
Therefore, the exhaust gas cleaning filter having good properties is preferably made of a material having a large crack limit, an excellent strength against thermal stress and a small pressure loss.
Recently, fuel previously including a fuel additive, or a device dropwise adding a fuel additive to a fuel is developed for controlling the amount of the particulate produced in the exhaust gas and the use thereof is increasing. Such a fuel additive has an effect of preventing the formation of soot in the burning of the fuel.
However, the formation of the particulate can not completely be controlled even by using such a fuel additive and hence the particulate is formed in the exhaust gas. Therefore, it is indispensable to use the exhaust gas cleaning filter.
In the conventional technique, the cordierite filter is generally adopted as a checkered honeycomb filter for cleaning the exhaust gas as previously mentioned. However, there is a problem that the amount of the particulate to be treated in one regeneration of the cordierite filter has a limit because the maximum service temperature in the filter is low. In this case, a large pressure loss is caused in the filter due to the deposition of the particulate, so that the combustion efficiency of the internal combustion engine lowers to degrade the fuel consumption.
And also, there is proposed a technique for regenerating the exhaust gas cleaning device by burning the particulate caught on the cordierite filter through a heating means for the filter. However, when such a greater amount of the particulate caught on the filter is burnt out by the heating means at once, a large change of the pressure loss is caused in the burning of the particulate in accordance with the heat conduction efficiency of the heating means, which gives incompatible feeling to a driver.
In FIG. 3 are shown experimental data illustrating a relation among pressure loss xcex94P (mmAq) and temperature T (xc2x0C.) and time t (min) in the conventional checkered honeycomb cordierite filter for cleaning the exhaust gas. In FIG. 3, symbol Po is a case of burning a fuel containing a fuel additive, wherein as the temperature T rises with the increase of engine revolution number (engine loading), the deposition of particulates begins to decrease on the border of a certain time. That is, the burning of the particulate is begun at a temperature of To=about 380xc2x0 C. to conduct the regeneration of the filter.
On the other hand, symbol Pn is a case of burning a fuel containing no fuel additive, wherein the pressure loss xcex94P in the filter continuously rises in proportion to the deposition of the particulate even when the temperature Tn rises with the increase of the engine revolution number (engine loading). As a result, the burning of the particulate at a temperature of Tn=about 380xc2x0 C. is not carried out different from the case of burning the fuel containing the fuel additive. Moreover, the beginning temperature of burning the particulates in case of using the fuel containing no fuel additive is generally about 630xc2x0 C.
In order to reduce the pressure loss produced in the cordierite filter, therefore, it is considered to finely set the cell structure Cs of the cordierite filter with reference to FIG. 2. For example, it is considered that the cell number Nc (cpi) is set to a large value, while the thickness dc of the cell wall is set to a small value.
In the conventional cordierite filter, however, there is a limit in the formation of the fine cell structure from a view point of the strength inherent to the cordierite. For example, it is possible to manufacture the cordierite honeycomb filter having the cell number Nc of more than 100 cpi, but when such a filter is used as a checkered honeycomb filter capable of efficiently burning the particulate, the cracking is caused in view of the crack limit of cordierite itself, so that the cell number Nc (cpi) can not be made more than 100 when the checkered honeycomb filter for cleaning the exhaust gas is made of cordierite.
In addition, if it is intended to burn the deposited particulates through only the temperature of the exhaust gas, there is caused an inconvenience that the honeycomb filter for cleaning the exhaust gas can not completely be regenerated because the exhaust gas may not rise to a temperature required for burning the particulate when the vehicle is frequently run on urban area at a low speed.
Under the above situations, it is an object of the invention to provide a regeneration system for an exhaust gas cleaning device disposed in an exhaust emission path of an internal combustion engine capable of controlling a change of pressure loss to a smaller value during the regeneration and conducting the complete regeneration of the filter even at a running state hardly raising the temperature of the exhaust gas by using an exhaust gas cleaning honeycomb filter made of a porous silicon carbide sintered body, which can be set to a fine cell structure and is high in the crack limit and excellent in the strength against thermal stress, together with a heating means for the filter when a fuel containing a fuel additive is used as a fuel for the internal combustion engine.
According to the invention, there is the provision of a regeneration system for an exhaust gas cleaning device disposed in an exhaust emission path of an internal combustion engine comprising an exhaust gas cleaning honeycomb filter for collecting particulates included in the exhaust gas and a heating means for the exhaust gas cleaning honeycomb filter, characterized in that said internal combustion engine uses a fuel containing a fuel additive, and said filter is a checkered honeycomb filter made of a porous silicon carbide sintered body and having a cell structure that a cell number per square inch is not less than 100 cells and a thickness of a cell wall is not more than 0.43 mm, and said heating means is selected from a heater and a glow plug.
In a preferable embodiment of the invention, the glow plug is a ceramic glow plug.
In another preferable embodiment of the invention, the filter has a total volume corresponding to xc2xc-2 times an engine swept volume of the internal combustion engine.
In the invention, the exhaust gas cleaning honeycomb filter is a checkered SiC honeycomb filter made of the porous silicon carbide sintered body, so that it is high in the crack limit and excellent in the strength against thermal stress as compared with those of the conventional cordierite filter and hence the durability of the filter is high. And also, the checkered SiC honeycomb filter has a cell structure that the cell number per square inch is not less than 100 cells and the thickness of the cell wall is not more than 0.43 mm (=17 mil), which can be made finer than that of the conventional cordierite filter, so that the pressure loss can be decreased as compared with that of the cordierite filter to improve the fuel consumption.
Furthermore, in the regeneration system according to the invention, the heater or the glow plug as the heating means for the exhaust gas cleaning filter is arranged ahead the filter in the exhaust gas cleaning device, so that even if the temperature of the exhaust gas is not raised to a level required for burning the particulate during the running of the vehicle on urban area at a low speed or the like, the particulate can be burned by heating the filter through such a heating means, and hence the regeneration of the filter can completely be attained.
In case of using the glow plug, the space in the exhaust gas cleaning device can be saved and power consumption can be decreased as compared with those in the use of the heater. As the glow plug, there are a metal glow plug and a ceramic glow plug. The use of the ceramic glow plug is favorable because the time reaching to a given temperature by heating is faster than that of the metal glow plug and a saturated temperature can be made higher. And also, the ceramic glow plug is small in the power consumption as compared with the metal glow plug and is high in the durability. Therefore, the more efficient regeneration of the filter can be attained by the use of the ceramic glow plug as compared with the use of the heater or the metal glow plug.
In the invention, the total volume of the exhaust gas cleaning filter is set to xc2xc-2 times the engine swept volume of the internal combustion engine because the total volume is dependent upon the engine swept volume. When the total volume of the filter is less than xc2xc times the engine swept volume, the sufficient filtering area can not be ensured and the pressure loss becomes larger to bring about the remarkable degradation of the fuel consumption, while when it exceeds 2.0 times, it is difficult to arrange the exhaust gas cleaning device inclusive of the filter in the exhaust emission path but also the thermal capacity becomes larger to delay reaction to the temperature of the exhaust gas to thereby lose a chance of obtaining a regeneratable temperature of the filter for burning the particulate.